Recently, a dye-sensitized photoelectric conversion device and a solar cell have been attracting attention as a next-generation solar cell. In a solar cell, a solvent is generally used as an electrolyte solvent. An organic solvent is not necessarily practical in view of stability of a cell. Consequently, an ionic liquid with thermal stability and non-volatility is attracting attention as a candidate for an electrolyte solvent. However, there is a problem that a solar cell with an ionic liquid has lower conversion efficiency than a solar cell with an organic solvent.
In a case where an ionic liquid is used as an electrolyte solvent, it is expected that the ionic liquid be quasi-solidified so as to ensure long-term stability of a cell. Examples of a technique for quasi-solidifying the ionic liquid include a chemical gel and physical gel. The chemical gel is a gelling method using a chemical reaction such as formation of a macromolecule compound. The physical gel is a quasi-solidification technique using a non-covalent interaction such as a hydrogen bond and a van der Waals force. Recently, researches on practical use of dye-sensitized solar cells using the chemical gel or the physical gel have been actively made.
Noteworthy is a method for gelling an ionic liquid with use of nanoparticles such as titanium oxide and carbon nanotubes. Researches have been made in order to apply the method to dye-sensitized solar cells (Non-patent Documents 1 and 2 and Patent Document 1)
As an example of application of the physical gel to dye-sensitized solar cells, there has been reported only an ionic liquid gel prepared with use of a low-molecular gelling agent (Non-patent Documents 3 and 4).
However, although a dye-sensitized solar cell including an ionic liquid quasi-solid electrolyte prepared with use of a low-molecular gelling agent realizes relatively excellent stability of a cell, conversion efficiency of such dye-sensitized solar cell is 5%, which is much lower than that of a system including an electrolyte made of an organic solvent. The reason is supposed to be prevention of ionic conduction by quasi-solidification.
Further, there is a case where iodine, lithium iodide, t-butylpyridine etc. is added in order to increase conversion efficiency of a solar cell. However, such addition greatly drops gelling ability of a low-molecular gelling agent. In fact, there is no example in which an ionic liquid electrolyte to which lithium iodide or t-butylpyridine is added is gelled with use of a low-molecular gelling agent. As described above, there is no suitable low-molecular gelling agent, which is supposed to be one reason why conversion efficiency of a solar cell is not increased.
Patent Document 1: Japanese Unexamined Patent Publication No. Tokukai 2005-93075
Non-patent Document 1: Wang et al., J. Am. Chem. Soc., 2003, 125, 1166
Non-patent Document 2: Usui et al., J. Photochem. PhotobioL. A: Chem., 2004, 164, 97;
Non-patent Document 3: Kubo et al., Chem. Commun., 2002, 374;
Non-patent Document 4: J. Phys. Chem. B, 2003, 107, 4374